Here's a really scary prompt: 'The isolated lighthouse on the stormy sea'. Picture a lighthouse keeper alone during a huge storm. The waves are crashing against the rocks below. The lighthouse keeper starts to see strange shapes in the rain - soaked windows. He hears banging on the door, but when he opens it, there's nothing there. As the night goes on, he realizes that something is trying to get in, and he's trapped in this small lighthouse with an unknown and terrifying presence.
Consider 'The attic in the old Victorian house'. The attic is filled with dusty old boxes and cobwebs. There's an old rocking chair that starts to rock on its own. A family moves into the house, and their young daughter is curious about the attic. One night, she goes up there and starts to feel a cold breeze. She hears a lullaby being sung softly, but there's no one there. This could be the start of a story about the secrets hidden in the attic and the ghosts that inhabit it.
A spooky prompt could be 'A haunted forest'. You start by describing the forest as dark and foreboding. The trees are gnarled and seem to reach out like twisted arms. There are strange noises like whispers in the wind. A character enters the forest, and as they do, they feel a sense of dread. Maybe they see a figure in the distance that disappears when they look closer. This can lead to a story about the character trying to find their way out while being haunted by the unknown entities in the forest.